The availability to obtain a GPS navigation solution is limited by an ability of a receiver to acquire the GPS signal in the presence of interference such as foliage attenuation encountered in forest, in-building attenuation cause by large structures, and multipaths caused by signal path reflections and obstructions. Particularly, attenuation encountered during in-building GPS reception, such as when GPS receivers are integrated into cell phones, limits the use of emergency GPS applications. These conditions make it desirable to fully exploit the available GPS signal strength. One factor limiting the acquisition of a GPS signal with weak signal levels is the processing gain in the presence of interference. This is especially critical for in-building and urban applications of GPS receivers.
Usually, it is desirable to maximize the coherent integration period to maximize the processing gain in a GPS receiver. The coherent integration period is typically limited to the data bit period. In acquisition of the P(Y) code signal, the non-repetitive nature of the code, and the coherency of the code to the GPS data message will precisely determine the bit boundaries relative to the start of the integrate and dump period. Due to the design of the GPS course acquisition (C/A) code, used by commercial GPS receivers, the code period of 1 ms is much less than the 20 ms period of the GPS data bit. As a result of the ambiguity of the code relative to the data bit boundary, bit synchronization is usually required as a necessary step after initial code acquisition. That is, bit synchronization is normally achieved after initial C/A code acquisition. However, bit synchronization is not required when the period of the code, such as the P(Y) code and the new L2 and L5 civil codes, is greater than or equal to the data bit period because the bit boundaries are not ambiguous after code acquisition. It is desirable to acquire the C/A code with the greatest possible coherent integration period. Due to integration across random data bit boundaries, a C/A code receiver incurs additional losses depending upon the relative phase of the integrate and dump clock and the start of a bit boundary. For the C/A code acquisition, the integration may start at some random bit boundary offset within a bit period. This bit synchronization offset leads to poor signal reception and disadvantageously requires bit synchronization after code acquisition.
For the C/A code, having a 1.0 ms code period, the processing gain is maximized by integration over the 20 ms bit period. In order to precisely integrate and dump over a 20 ms bit period, the integration period must be aligned, that is, synchronized, to bit boundaries. Thus, bit boundary determination during signal acquisition can improve receiver sensitivity to weak signals. For the C/A code, these boundaries are known only to within some multiple of the 1 ms C/A code periods. Consequently, the start of an integration period can be offset by as much as 10 ms from the start of a bit boundary when the 20 ms integration period is used. Consequently, the acquisition process is usually limited to small integration periods to avoid increased signal losses due to lack of bit boundary synchronization prior to carrier tracking. As the C/A code is initially acquired by conventional code phase determination, bit boundaries will not be synchronized with the integration period, and hence conventional methods require noncoherent integration to mitigate these signal losses. Noncoherent integration results in reduced receiver sensitivity during signal acquisition.
While wireless assisted GPS aiding has been to reduce initial time uncertainty to enable bit boundary determination during acquisition, thereby maximizing processing gain, such aiding disadvantageously requires reliance on a wireless network. Conventional signal acquisition methods do not eliminate the need for wireless assistance to resolve bit boundary offsets and do not maximize coherent integration period, and hence, do not maximize the processing gain during acquisition resulting in poor reception of weak signals during signal acquisition. These and other disadvantages are solved or reduced using the invention.